The present invention relates to a plasma addressed display device, and more particularly to a technique for adaptively controlling a voltage to be applied to each discharge channel by line-sequential scanning.
A plasma addressed flat panel employing a plasma cell for addressing of a display cell is known from Japanese Patent Laid-open No. 4-265931, for example. As shown in FIG. 6, such a flat panel 0 is composed of a display cell 1, a plasma cell 2, and a common intermediate substrate 3 interposed between the display cell 1 and the plasma cell 2. The intermediate substrate 3 is formed from a very thin glass plate or the like, which is called a microsheet. The plasma cell 2 is configured by a lower substrate 4 bonded to the intermediate substrate 3 with a gap defined therebetween. The gap is filled with an ionizable gas. A plurality of parallel stripes of discharge electrodes 5 are formed on the inside surface of the lower substrate 4. The discharge electrodes 5 may be printed and baked on the flat substrate 4 by screen printing or the like. A plurality of barrier ribs 6 are formed on the inside surface of the lower substrate 4 in such a manner that each barrier rib 6 partitions any adjacent pairs of the discharge electrodes 5, thereby dividing the gap filled with the ionizable gas into a plurality of discharge channels 7. The barrier ribs 6 may also be printed and baked by screen printing or the like. The top of each barrier rib 6 is in contact with the lower surface of the intermediate substrate 3. Each pair of discharge electrodes 5 function as an anode A and a cathode K, between which a plasma discharge is generated. The intermediate substrate 3 and the lower substrate 4 are bonded to each other through a glass frit 8 or the like.
On the other hand, the display cell 1 is configured by a transparent upper substrate 9. The upper substrate 9 is bonded to the upper surface of the intermediate substrate 3 through a sealing member 10 or the like, so that a given gap is defined between the upper substrate 9 and the intermediate substrate 3. This gap is filled with an electro-optic substance such as a liquid crystal 11. A plurality of parallel stripes of signal electrodes 12 are formed on the inside surface of the upper substrate 9. The signal electrodes 12 perpendicularly intersect the discharge electrodes 5, so that a plurality of pixels forming a matrix are defined at the intersections between the signal electrodes 12 and the discharge channels 7.
In the plasma addressed flat panel 0 having such a configuration, the rows of discharge channels 7 in which the plasma discharge is generated are line-sequentially switched to be scanned, and in synchronism with this line-sequential scanning an image signal is applied to each signal electrode 12 of the display cell 1, thereby displaying an image. When the plasma discharge is generated in each discharge channel 7, the interior of each discharge channel 7 uniformly becomes an anode potential to thereby select pixels row by row. That is, each discharge channel functions as a sampling switch. When an image signal is applied to each pixel in an on-state of the plasma sampling switch, sampling is performed to allow on/off control of each pixel. Even after the plasma sampling switch goes off, the image signal is held in each pixel as it stands.
FIG. 7 is a block diagram showing a general configuration of a plasma addressed display device employing the flat panel 0 shown in FIG. 6 as a display. The display cell 1 and the plasma cell 2 are laminated together to form an effective screen 20. A signal circuit 21 is connected to the signal electrodes 12 of the display cell 1 to supply an image signal to each signal electrode 12. The anodes A of the plasma cell 2 are commonly connected to a grounding terminal of a main power supply 22. A negative terminal voltage of the main power supply 22 is represented by -Vk. The cathodes K are connected to a drive circuit 23. The drive circuit 23 is configured by a plurality of switching elements such as transistors. When the switching elements sequentially go on/off, the plasma discharge in each discharge channel is moved from an upper portion to a lower portion of the screen 20. A discharge current flows from the anode A to the cathode K in each discharge channel, and then passes through the corresponding switching element to a constant-current circuit 24. The constant-current circuit 24 controls a cathode potential Vo to limit the discharge current to a constant value. The constant-current circuit 24 is supplied with a voltage Va from an auxiliary power supply 25.
FIG. 8 is a circuit diagram showing an illustrative configuration of the constant-current circuit 24 shown in FIG. 7. The constant-current circuit 24 shown in FIG. 8 is a current mirror circuit composed mainly of a pair of transistors Tr0 and Tr1. The cathode potential Vo is controlled so that the same current as the current determined by the voltage Va of the auxiliary power supply and a resistance R flows into the transistor Tr0. A diode Di serves as a bypass for supplying a constant current to the transistor Tr0 during a period when the switching elements are off (i.e., during a nondischarge period). In the case of this constant-current circuit, the current flows from the diode Di to the transistor Tr0 during the nondischarge period, so that the cathode potential Vo becomes -Vk+Va.
The operation of the constant-current circuit shown in FIG. 8 will now be described in brief with reference to FIG. 9. At a discharge timing when each switching element of the drive circuit goes on, the above-mentioned potential -Vk+Va is initially applied to the cathode. This applied voltage is the same to all the discharge channels. When the discharge starts, the discharge current flows into the transistor Tr0, and the cathode potential Vo changes according to the magnitude of the flowing discharge current. An amount of change in the cathode potential Vo varies with time and variations in characteristics between the discharge channels.
As mentioned above, the conventional plasma addressed display device employs an active load such as the constant-current circuit as means for limiting the discharge current. In this case, when the discharge starts, the potential during the nondischarge period is initially applied between the anode and the cathode. This applied voltage is fixed to the constant value -Vk+Va by the main power supply and the auxiliary power supply. This applied voltage must be preliminarily set to a large value in consideration of variations in discharge characteristics between the discharge channels and aged changes in the discharge characteristics. As a result, until the discharge current reaches a constant value by the operation of the constant-current circuit, an excess voltage higher than a voltage required by the discharge channel is applied between the anode and the cathode. Accordingly, a local arc discharge is generated to damage the electrodes and resultantly shorten the life of the flat panel.